Portable communication devices, such as cellular telephones, use one or more power amplifiers to amplify an information signal prior to transmission. One of the technologies used to fabricate a power amplifier for a portable communication device uses the gallium arsenide (GaAs) material system. A typical GaAs power amplifier operates using a reference voltage signal on the order of approximately 2.8 volts (V) to bias the transistors that make up the power amplifier. Biasing the transistors is important to establish the proper quiescent current. Further, the bias current should be consistent across different parts and over temperature and process variations. However, generating the reference voltage signal in the GaAs material system provided challenges. As a result of these challenges, a nominal 2.8V reference voltage signal is typically provided by circuitry separate from the GaAs material system from which the power amplifier is fabricated. Unfortunately, this adds cost and complexity to the portable communication device.
In addition, to reduce power consumption in modern portable communication devices, virtually all of the other components in the portable communication device are being designed and manufactured to operate at lower voltage and current levels. For example, most modern components of a portable communication device are being designed to operate at a nominal voltage of approximately 1.8V. This nominal 1.8V may vary between approximately 1.2V and 1.8V. This further complicates the ability to provide the GaAs-based power amplifier with the desired nominal 2.8V reference voltage signal.
Further, portable communication device manufacturers desire the ability to supply a voltage control signal to set the bias current of a power amplifier based on the power level to be transmitted.
Therefore, it would be desirable to have a bias circuit that overcomes these challenges.